MCAS was introduced to counteract the pitch up effect of the LEAP-1B engines at high AoA. The engines were both larger and relocated slightly up and forward from the previous NG CFM56-7 engines to accomodate their larger diameter. This new location and size of the nacelle causes it to produce lift at high AoA; as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall.

On the face of it this seems like a sensible, beneficial system. However following the accident to Lion Air MAX-8 PK-LQP on 29 October 2018, shortly after take-off, in which it appears that the Captains AoA sensor was faulty, it is believed that the MCAS used this erroneous AoA data to command nose down stabiliser which was not counteracted sucessfully by the crew until the aircraft impacted the water.

As details emerged about the similarities between the two accidents, the 737 MAX was grounded by many countries unilaterally. The FAA followed suit on 13 March. Boeing issued the following statement: “After consultation with the US FAA, the US NTSB, and aviation authorities and its customers around the world, Boeing has determined—out of an abundance of caution and in order to reassure the flying public of the aircraft’s safety—to recommend to the FAA the temporary suspension of operations of the entire global fleet of 371 737 MAX aircraft. Boeing makes this recommendation and supports the decision by the FAA.”

All of the information, photographs & schematics from this website and much more is now available in a 374 page printed book or in electronic format.

The Proposed Fix

Boeing have been working on a software modification to MCAS since the Lion Air accident. Unfortunately although originally due for release in January it was not released due to both engineering challenges and differences of opinion among some federal and company safety experts over how extensive the changes should be.

As of 21 March 2019 the word was that there were three significant changes to MCAS software being worked on by Boeing:

To give the system input from both angle-of-attack sensors, Currently MCAS only uses data from the angle of attack sensor on the side of the active FCC, (see AoA source below)

To limit how much MCAS can move the horizontal stab. In its original report, Boeing said that MCAS could move the horizontal stabilizer a maximum of 0.6 degrees. However, after the Lion Air crash, it told airlines that MCAS could actually push the controls 2.5 degrees, or half the physical maximum. Boeing reportedly increased the limit because flight tests showed that a more powerful movement was needed to counteract an impending stall.

To limit MCAS to operate only for one cycle, rather than multiple. At present it will pause for 5 seconds and repeat for as often as it senses the impending stall condition is present.

Furthermore there is expected be:

Changes to the on board displays, expected to be AoA displays and AoA disagree indications;

FAA Actions

On March 11, 2019, the FAA issued a Continued Airworthiness Notification to the International Community (CANIC), providing information regarding FAA continued operational safety activity related to the Boeing 737-8 and Boeing 737-9 (737 MAX) fleet. (available here)

Situation description: Following the accident of an Ethiopian Airlines Boeing Model 737-8 airplane on March 10, 2019, the National Transportation Safety Board (NTSB) as the accredited representative, and the FAA as Technical Advisors, are supporting the Ethiopian Accident Investigation Bureau. The FAA has dispatched personnel to support the investigative authorities in determining the circumstances of this event. All data will be closely examined during this investigation, and the FAA will take appropriate action if the data indicates the need to do so.

External reports are drawing similarities between this accident and the Lion Air Flight 610 accident on October 29, 2018. However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions.

Following the Lion Air Flight 610 accident, the FAA has completed these activities in support of continued operational safety of the fleet:

- Boeing’s completion of the flight control system enhancements, which provide reduced reliance on procedures associated with required pilot memory items. The FAA anticipates mandating these design changes by AD no later than April 2019.

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

To summarise; MCAS will trim the Stabilizer down for 10 seconds (2.5 deg nose down) and pause for 5 seconds and repeat if the conditions (high angle of attack, flaps up and autopilot disengaged) continue to be met. Using electric pitch trim will only pause MCAS for 5s; to deactivate it you need to switch off the STAB TRIM SUTOUT switches.

The MAX probes on the LHS. The AoA vane is the bottom of the three probes.

The AoA source

I had assumed that the AoA source for MCAS was always the Captains AoA probe but the following explanation from an engineer suggests that it alternates between AoA probes each flight:

MCAS is implemented within the two Flight Control Computers (FCCs). The Left FCC uses the Left AOA sensor for MCAS and the Right FCC uses the Right AOA sensor for MCAS. Only one FCC operates at a time to provide MCAS commands. With electrical power to the FCCs maintained, the unit that provides MCAS changes between flights. In this manner, the AOA sensor that is used for MCAS changes with each flight.

There is an airline option to have AoA data displayed on the PFDs, as far as I know this option has only been taken by Southwest and American Airlines. Crucially, there is an AoA Disagree Alert which displays the word "AOA DISAGREE" in amber at the bottom left of the PFD if the AoA vanes disagree by more than 10 degrees for more than 10 continuous seconds.

FAA Emergency AD 2018-23-51 - SUMMARY: We are adopting a new airworthiness directive (AD) for all The Boeing Company Model 737-8 and -9 airplanes. This emergency AD was sent previously to all known U.S. owners and operators of these airplanes. This AD requires revising certificate limitations and operating procedures of the airplane flight manual (AFM) to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions. This AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer. We are issuing this AD to address the unsafe condition on these products..